1. Field of the Invention
The present invention relates to oxidation catalysts, and more particularly to mordenite oxidation catalysts which are useful for producing maleic anhydride from C.sub.4 to C.sub.10 hydrocarbons.
2. The Prior Art
Essentially all of the catalysts and methods disclosed in the prior art are for producing maleic anhydride from C.sub.4 and higher hydrocarbons employ oxidation catalysts containing vanadium in a valence state of less than +5. One method of forming such catalysts is to impregnate a catalyst base with a vanadium compound in which the vanadium has a valence of less than +5. Another more desirable method involves impregnating the catalyst with a vanadium compound which has a vanadium in the +5 valence state and then reducing the vanadium from the +5 valence state to a valence less than +5.
Several references disclose oxidation catalysts containing vanadium-phosphorus mixed oxide catalysts and methods of preparing the same. For example, U.S. Pat. No. 4,179,404 discloses a process for preparing vanadium-phosphorus containing oxidation catalysts which consists of reducing pentavalent vanadium to a valence of less than +5 with a trivalent phosphorus compound. The phosphorus compound is employed in a concentration of from about 75 to 90 percent of the stoichiometric amount necessary to reduce the vanadium to a valency of from +5 to +4.
U.S. Pat. No. 4,153,577 discloses a catalyst complex useful for the partial oxidation of alkanes to the corresponding anhydrides in a vapor phase reaction. The oxidation catalyst used is a reduced vanadium and phosphorus mixed oxide catalyst containing either transition metals, Group IIA metals or rare earth metals.
Another oxidation catalyst suitable for preparing maleic anhydride from normal C.sub.4 hydrocarbons is disclosed in U.S. Pat. No. 4,123,388 which relates to a vanadium, phosphorus, copper mixed oxide complex containing an alkali or alkaline earth metal. In addition, tin is described as a desirable metal for incorporating into the catalyst.
U.S. Pat. No. 4,092,269 relates to vanadium-phosphorus oxidation catalysts wherein at least 20 atom percent of the vanadium is in the tetravalent state. A pore modification agent selected from polymeric materials, cellulosic materials, monosaccharides, etc. is added to the catalyst to provide pore diameters between 0.8 to 10 microns. The catalyst is described as useful for the conversion of aliphatic hydrocarbons to maleic anhydride.
Finally, U.S. Pat. No. 3,915,892 discloses a method of preparing a vanadium-phosphorus mixed oxide oxidation catalyst utilizing three bulk phase transitions, wherein the average valence of vanadium is maintained in the range of 4.1 to 4.5 and in addition a partial pressure of oxygen is maintained in contact with the mixed oxides formed.
As can readily be determined from the above, there is an ongoing effort to develop oxidation catalysts for preparing maleic anhydride from alkanes and olefins.
Accordingly, it is an object of the present invention to provide an improved oxidation catalyst for oxidizing saturated and unsaturated hydrocarbons to maleic anhydride.
Another object of the present invention is to provide an oxidation catalyst having a large surface area which is useful for producing maleic anhydride and to provide a method of preparing the same.
A further object of the present invention is to provide a method for obtaining improved yields and selectivity of maleic anhydride and, in addition, improvements in catalyst stability.
These and other objects are accomplished according to the present invention by oxidizing either a saturated or unsaturated hydrocarbon having from four to ten carbon atoms in the presence of an oxidation catalyst comprising the oxides of vanadium, phosphorus, tin on a hydrogen mordenite support.